1. Field of the Invention
This invention relates generally to a servo for a recording and/or reproducing apparatus which rotates a record carrier, and more particularly, to a servo for displacing a movable element in a transducer head assembly of the apparatus in a sense to eliminate deviations of the transducer from a predetermined condition with respect to the carrier resulting from fluctuations of the carrier upon rotation thereof.
2. Description of the Prior Art
It has ben proposed to record and/or reproduce optical signals from a rotating carrier, for example, a disc, with apparatus having servo mechanisms for displacing a mirror and lens in a transducer head assembly of the apparatus to keep a light beam focused on a predetermined track on the carrier. One example of such apparatus, a video disc recorder, will be used to illustrate the invention.
The proposed video disc recorder operates with a carrier record disc on which video program signals are optically recorded on successive turns of a spiral track on a surface of the record disc. In order to provide a video program of reasonable duration, for example, one half hour, on a disc of reasonable size, for example from 10 cm. to 30 cm. useful diameters, the optical video program signals must be recorded with an extremely high density of, for example, 500 successive turns of the track per radial millimeter of the video disc. An extremely accurate way of following the track is therefore required to obtain the video program signals.
It is therefore proposed to provide a transducer with a laser light which can be focused into a sufficiently tight beam for transducing the optical signals from the track on the disc into electrical video program signals. The transducer is mounted in a head assembly adjacent the disc and, as the disc is rotated to progress each turn of the track past the head assembly, the head assembly is moved radially of the disc to approximately scan the successive turns in the spiral track on the disc with the laser light beam.
The disc is rotated at a high speed in order to obtain sufficient information from the optically recorded signals on the disc for reproducing the video program. (For convenience, the high speed is selected to correspond to the frame frequency of the video picture or 30 rotations per second in the present NTSC video system and 25 rotations per second in the PAL video system.) At the high rotational speed of the disc, the head assembly cannot move accurately enought to direct the light beam onto the narrow track of the high density recording.
Moreover, the position of the track may fluctuate radially of the head assembly from eccentricity or vibration in the arrangement for rotating the disc. For example, a defect in centering a central hole in the disc which is used in the disc rotating arrangement will make each turn of the track eccentric of the disc rotation to cause the position of the track to fluctuate relative to the head assembly at a frequency corresponding to the disc rotation. Similarly, warping or vibration of the disc normal to its plane of rotation will move the disc toward and away from the head assembly, again at a frequency corresponding to the disc rotation, so that the light beam may be inadequately focused on the track of the disc. If an eccentricity in the path of a track relative to the head assembly causes the transducer to scan from one turn of the track to another, the continuity of the video signals will be lost and, if the disc moves too far toward or away from the head assembly, the light beam may focus on more than one turn of the track to produce cross talk between the video signals recorded on the adjacent turns. Both results are clearly undesirable. The transducer therefore must have another way of maintaining proper scan and focus conditions.
It has therefore been proposed to provide the laser transducer with movable mirror and lens elements and servo mechanisms for moving the elements to keep the transducer in the condition with the laser light beam focused on just one turn of the track. More specifically, the transducer directs a lseer light beam onto a half mirror, and from the half mirror to the movable mirror and through the movable lens to the disc. Light reflected from the optical signals recorded on the disc then passes back through the lens to the movable mirror and the half mirror, the reflected light, however, passing through the half mirror to a photodiode for transducing the optical signal into an electrical signal. The servo for moving the movable mirror causes the laser beam to scan the track and the servo for moving the lens maintains the focus of the laser beam on the track.
Inasmuch as rotation of the video disc introduces fluctuations in the position of the disc relative to the head assembly both radially of the disc and normal to its plane of rotation at a fundamental frequency corresponding to the rotational speed of the disc, the movable mirror for scanning the track and the movable lens for focusing the light beam on the track have heretofore been designed with a mechanical resonant frequency at the speed of rotation of the disc. The servos then have a most ready response at the fundamental frequency as indicated by a peak in the gain-frequency characteristic of the servos at the fundamental frequency.
The servos necessarily have some delay in moving the mirror or the lens into the proper scanning and focusing condition. At some high frequency of fluctuation in the track position relative to the head assembly, therefore, the response of the servos will be 180.degree. delayed from that desired. In this condition, the servos will introduce an undesirable oscillation if their gain is greater than one. It has therefore been the practice to provide servos having a peak response at the fundamental frequency corresponding to the speed of rotation of the disc from the mechanical resonant frequency of the movable element of the servo (the mirror or lens) and a decreasing response for higher frequencies so that the gain or response of the servo will be low, for example, less than one, when the frequency of the carrier fluctuation is high enough to cause the response of the servo mechanism to be 180.degree. delayed therefrom. For example, the servo may have an overall gain-frequency characteristic having a peak response at 30 Hz corresponding to the 30 revolutions per second of the disc for the NTSC system from the mechanical resonant frequency and a gain which decreases at 20 dB/Dec for higher frequencies so that the gain will be less than one when the frequency of carrier fluctuations is high enought that the response of the servo is 180.degree. delayed therefrom.
Although the arrangement described may be satisfactory for accommodating fluctuations in the disc at the fundamental frequency corresponding to the speed of rotation of the disc, the actual fluctuations in the disc occur not only at the fundamental frequency, but also at harmonics thereof. With the response of the servo being reduced at higher frequencies, however, the response of the servo may be insufficient to accommodate the fluctuations in the disc at the harmonics of the fundamental frequency. The lens and mirror then will not move sufficiently to maintain the desired scanning and focusing conditions.
Although the transducer arrangement just described transduces recorded optical signals on the disc into electrical signals, it will be understood that a similar arrangement could be used to laser scribe the optical signals on the disc from electrical signals. Such a recording arrangement will also require a way of maintaining the focusing and scanning conditions on the desired track which may be carried out with a servo system similar to that just described. It will therefore be understood that the invention herein described has utility with both recording and reproducing apparatus.